The present invention relates to a process for the preparation of an alkyl methacrylate. In a specific aspect, the invention relates to a process for preparing methyl methacrylate.
Methyl methacrylate is prepared in industry mainly by the acetone cyanohydrin process. This process presents disadvantages in that large quantities of waste sulphuric acid and ammonium bisulphate are produced, and these by-products must be discharged or worked up for reuse. Another by-product, HCN, is highly toxic and raises difficult problems of storage and transportion. As concerns about the environment have increased, considerable research has been devoted to finding alternative processes which do not present these disadvantages.
One possible alternative process, described in 1964 by Y. Sakakibira in Bull. Chem. Soc. Japan 37, 11 (1964) 1601-1609, involves the reaction of propyne with carbon monoxide and alkanol in the presence of a carboxylation catalyst. Although this process has been known for a long time and has attracted a considerable amount of interest, it has never been commercialized.
A factor inhibiting the commercial exploitation of the carboxylation process has been the unavailability of large quantities of a suitable low-priced propyne feed.
Many processes have been described for the preparation of propyne. For example, the chapter "Methylacetylene" in Kirk-Othmer's Encyclopedia of Chemical Technology, 2nd ed., Volume Supplement (1971), pages 547 to 556, refers to various processes including the dehydrohalogenation of propylene dibromide, the hydration of magnesium carbide, the reaction of sodium acetylide and dimethylsulphate in liquid ammonia, and a variety of pyrolysis or cracking methods.
European patent application publication number EP-A-0190473 discloses a process for the preparation of alkyl acrylates, such as methyl methacrylate, by the carboxylation of propadiene. Example 10 in the specification describes an experiment in which methyl methacrylate is prepared by reacting a mixture of propadiene and propyne with carbon monoxide and methanol in the presence of a relatively inactive carboxylation catalyst. Both the propadiene and the propyne are converted into methyl methacrylate. Surprisingly, it has now been found that propadiene poisons carboxylation catalysts in the carboxylation of propyne and methanol to give methyl methacrylate. Moreover, the poisoning effect of propadiene appears to increase as the intrinsic activity of the carboxylation catalyst for propyne carboxylation increases.
It is thus an object of the invention to provide a process for the preparation of an alkyl methacrylate which can be carried out at industrial scale and at low cost. It is a further object of the invention to provide a process which uses large quantities of propyne in a quality in which it may be easily supplied. Finally, it is an object of the invention to provide an industrial scale process which can make use of high activity carboxylation catalysts.